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What's the difference between turbo lag and boost threshold?

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The automotive enthusiast community occasionally mixes and matches a few of its key terms. Wheels versus rims, for example. Another example can be found when dealing with engines that breathe through a bit of forced induction.

On a turbocharged engine, there are two physical concepts that can often confuse folks. In one corner we have boost threshold. In the other you’ll find turbo lag. Jason Fenske from Engineering Explained is ready to set us straight on the meaning of each.

Let’s start with boost threshold. That’s the term to describe the engine speed required to produce enough exhaust gas flow that in turn generates the boost a turbo needs to start spinning and feeding air into the engine. So it’s a level of engine speed where the turbo has positive manifold pressure. It’s almost like a changeover point for a VTEC engine.

On the other side of the turbocharger discussion, you’ll find turbo lag. This describes the amount of time between the point when the throttle is opened and the turbo spools up. You’re already operating the engine at a point past the boost threshold though, so that is not a factor in the turbo lag equation. Instead, turbo lag can be related to engine tuning, intake design, backpressure, and the size of the turbocharger itself. A larger turbo simply takes longer to spool up and deliver more boost.

Boost threshold is the point at which your turbo is primed for spinning because the engine speed is sufficient to create positive pressure. Turbo lag is the delay in time from that turbo creating more boost once you’ve crossed the boost threshold and have gone from a closed throttle to an open one.